1. Technical Field
This invention relates generally to double acting hydraulic jars for use in drilling equipment and, in particular, to an improved mechanism for actuating the double acting hydraulic jar that is compact in size so as to increase the diameter of a drilling fluid bore extending through the jar and to increase the allowable overpull during actuation.
2. Description of the Related Art
Drilling jars have long been known in the field of well drilling equipment. A drilling jar is a tool employed when either drilling or production equipment has become stuck to such a degree that it cannot be readily dislodged from the wellbore. The drilling jar is normally placed in the pipe string in the region of the stuck object and allows an operator at the surface to deliver a series of impact blows to the drill string via a manipulation of the drill string. Hopefully, these impact blows to the drill string dislodged the stuck object and permit continued operation.
Drilling jars contain a sliding joint which allows relative axial movement between an inner mandrel and an outer housing without allowing rotational movement. The mandrel typically has a hammer formed thereon, while the housing includes an anvil positioned adjacent the mandrel hammer. Thus, by sliding the hammer and anvil together at high velocity, they transmit a very substantial impact to the stuck drill string, which is often sufficient to jar the drill string free.
Often, the drilling jar is employed as a part of the bottom hole assembly during the normal course of drilling. That is, the drilling jar is not added to the drill string once the tool has become stuck, but is used as a part of the string throughout the normal course of drilling the well. Thus, in the event that the tool becomes stuck in the wellbore, the drilling jar is present and ready for use to dislodge the tool.
However, since the drilling jar forms a portion of the drill string, then it must also include provision for passing drilling fluid therethrough. For example, drilling fluid is ordinarily circulated through an inner bore extending longitudinally through the drill string, out through the drill bit, and then up through the annulus formed by the wellbore and drill string. The drilling fluid is used to cool the drill bit, remove cuttings, and prevent "blowouts." A large volume of this drilling fluid is, therefore, passed through the longitudinal bore within the drill string. Clearly, with a larger diameter bore, more drilling fluid can be passed therethrough and the cooling and cutting removal is more efficiently performed. A drilling jar, however, differs substantially in mechanical complexity from the remainder of the drill string. This mechanical complexity necessarily results in a reduced diameter bore through the drilling jar, which, in turn, limits the flow of drilling fluid to the drill bit.
For example, U.S. Pat. No. 4,361,195, issued Nov. 30, 1982 to Robert W. Evans, describes a double acting drilling jar that has a reduced diameter longitudinal bore. In particular, the '195 patent describes an annular tripping valve that cooperates with a pair of control arms to provide this "double action." This mechanism, however, consumes a substantial diametric segment of the drilling jar, reducing the diameter of its internal longitudinal bore.
Further, the control arms of the '195 patent interact with the same control surfaces of the tripping valve to control both downward and upward jarring action. Accordingly, the same degree of movement between the mandrel and housing, and thus the same time delay, is present for actuating both upward and downward jarring. In some applications it is advantageous to have a different time delay associated with upward jarring than with downward jarring. The apparatus of the '195 patent has no such provision.
The present invention is directed to overcoming or minimizing one or more of the problems discussed above.